In the automobile industry, there is a continual need to lighten vehicles, resulting in a search for steels of higher yield strength or tensile strength. Thus, high-strength steels have been proposed that contain microalloying elements. Hardening is obtained at the same time by precipitation and by refinement of the grain size.
With the objective of obtaining even higher strength levels, TRIP steels have been developed that exhibit advantageous combinations of properties (strength/deformability). These properties are attributed to the structure of such steels, consisting of a ferrite matrix containing bainite and residual austenite phases. In hot-rolled sheet, the residual austenite is stabilized thanks to an increase in the content of elements such as silicon and aluminium, these elements retarding the precipitation of carbides in the bainite. Cold-rolled sheet made of TRIP steel is manufactured by reheating the steel, during the annealing, into a region where partial austenization occurs, followed by rapid cooling in order to avoid the formation of pearlite and then an isothermal soak in the bainite region: one portion of the austenite is converted to bainite while another portion is stabilized by the increase in carbon content of the residual austenite islands. Thus, the initial presence of ductile residual austenite is associated with a high deformability. Under the effect of subsequent deformation, for example during a drawing operation, the residual austenite of a part made of TRIP steel is progressively transformed to martensite, resulting in substantial hardening. A steel exhibiting TRIP behaviour therefore makes it possible to guarantee a high deformability and a high strength, these two properties usually being mutually exclusive. This combination provides the potential for high energy absorption, a quality typically sought in the automobile industry for impact-resistant parts.
Carbon plays an important role in the manufacture of TRIP steels: firstly, its presence in sufficient quantity within the residual austenite islands is necessary so that the local martensitic transformation temperature is lowered to below the ambient temperature. Secondly, it is usually added in order to increase the strength inexpensively.
However, this addition of carbon must remain limited in order to guarantee that the weldability of the products remains satisfactory, otherwise the ductility of welded assemblies and the cold cracking resistance are reduced. What is therefore sought is a manufacturing process for increasing the strength of TRIP steel sheet, in particular to above about 900-1100 MPa for a carbon content of around 0.2% by weight, without the total elongation being reduced to below 18%. An increase in strength of more than 100 MPa over the current levels is desirable.
It is also desirable to obtain a process for manufacturing hot-rolled or cold-rolled steel sheet which is largely insensitive to small variations in the industrial manufacturing conditions, in particular to temperature variations. Thus, it is sought to obtain a product characterized by a microstructure and mechanical properties that are largely insensitive to small variations in these manufacturing parameters. It is also sought to obtain a very tough product offering excellent fracture resistance.